Washington: Donald Trump demanded Michael Flynn's resignation as national security advisor over "eroding trust" and not due to any legal issues, the White House has said, shielding the US President from the raging scandal over his top aide's contacts with Russia.

"We have been reviewing and evaluating this issue with respect to General Flynn on a general basis for a few weeks trying to ascertain the truth," White House Press Secretary Sean Spicer told reporters.

The White House admitted that Trump was told three weeks ago that Flynn may have misled colleagues about his Kremlin contacts ahead of the inauguration on 20 January.

"We got to a point not based on a legal issue, but based on a trust issue with the level of trust between the president and General Flynn had eroded to the point where he felt he had to make a change. The president was very concerned that General Flynn had misled the vice president and others," he said.

Flynn, 58, resigned on Tuesday after he apologised to Trump and Vice President Mike Pence for not providing them all the information about his call with the Russian Ambassador to the US Sergei Kislyak before Trump's inauguration.

Michael Flynn. AP

Spicer said Trump was also very concerned in light of the sensitive subjects dealt with by the national security advisors with respect to China, North Korea and the Middle East, that he must have complete and unwavering trust for the person in that position.

"The evolving and eroding level of trust as a result of this situation and a series of other questionable instances is what led the president to ask for General Flynn's resignation," he said.

Spicer said immediately after the Department of Justice notified the White House counsel of the situation, the counsel briefed the President and a small group of his senior advisors.

"The White House counsel reviewed and determined that there is not an illegal issue, but rather a trust issue. During this process, it's important to note the President did not have his attorney general, Jeff Sessions, who he trusts immensely, approved by the Senate," he said.

"When the President heard the information as presented by White House counsel, he instinctively thought that General Flynn did not do anything wrong and the White House counsel's review corroborated that.

It is not ordinary for an incoming national security advisor to speak with his counterparts about the issues of concern to them. In fact, he spoke with over 30 of his counterparts throughout the transition," Spicer said.

"The issue here was that the President got to the point where General Flynn's relationship — misleading the vice president and others or the possibility that he had forgotten critical details of this important conversation had created a critical mass and an unsustainable situation. That's why the President decided to ask for his resignation and he got it," he said.